Catalytic hydrogenolysis (hydrocracking) of polyol is a process whereby polyols such as sugars, glycerol and glycols are reacted with hydrogen to produce other polyols. The polyols so produced often comprise a mixture of several polyols having a lower average molecular weight than the starting material. The impurity of the polyol product mixture (derivatives) presents a problem for sale and use of the product.
The conversion of polyols such as sugars and glycerol to polyhydric alcohols such as propylene glycol and ethylene glycol by hydrogenolysis or by hydrocracking results in formation of not only these alcohols, but several other unwanted products, such as 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol and 2,4-pentanediol. These products are recovered with the propylene glycol and ethylene glycol. Due to the similarity in boiling points, these diols are very difficult to separate from propylene glycol by distillation. For instance, in hydrocracking of higher carbohydrates such as sorbitol to produce propylene glycol, typically 3-5% by weight of 2,3-butanediol is produced in addition to 1,2-butanediol, ethylene glycol and 1,3-butanediol. Table 2 provides a list of polyols produced by hydrocracking of sorbitol as described in U.S. Pat. No. 4,935,102, which is incorporated herein by reference in its entirety. The boiling points of these components as shown in Table 1 are very close to one another such that in a rectification column, either under atmospheric pressure, reduced pressure or at an elevated pressure, the separation of substantially pure propylene glycol is difficult to attain.
TABLE 1Polyols produced by Hydrocracking ofSorbitol (U.S. Pat. No. 4,935,102)WeightBoiling Point,CompoundPercent° C.2,3-Butanediol3.5182Propylene glycol16.51871,2-Butanediol2.0192Ethylene glycol25.21981,3-Butanediol2.72062,3-Hexanediol—2061,2-Pentanediol—2101,4-Pentanediol—2201,4-Butanediol2.12301,5-Pentanediol0.1242Diethylene glycol2.22451,6-Hexanediol—250Triethylene glycol2.1285Glycerol38.82901,2,4-Butanetriol4.8190/18 mm100.00
The differences in volatility of propylene glycol compared to 2,3-butanediol and 1,2 butanediol are very small. As shown in Tables 2 and 3, for separation of these compounds from a mixture by distillation, the number of plates required to achieve 99% purity is very large; thus very tall distillation columns (55 trays for 2,3-Butanediol and 88 trays for 1,2-Butanediol) and high energy inputs are required.
TABLE 2Theoretical and Actual Plates Required vs. Relative volatilityfor Propylene Glycol - 2,3-Butanediol Separation.Actual Plates, 75%Relative VolatilityTheoretical PlatesEfficiency1.2541551.3531421.4525341.5023311.701824
TABLE 3Theoretical and Actual Plates Required vs. Relative volatilityfor Propylene Glycol - 1,2-Butanediol Separation.Actual Plates, 75%Relative VolatilityTheoretical PlatesEfficiency1.1566881.523312.014193.09123.5811
The processes involved in the hydrogenolysis of glycerol may be carried out by any of the known routes. These include heterogeneous metal catalysts, such as those described in U.S. Pat. No. 6,479,713, PCT patent application WO 2005/051874, US patent application 2005/0244312 or referred to in Catalysis Communications 6 (2005) 645-649 or Journal of Catalysis 240 (2006) 213-221, each of the contents of the entirety of which are incorporated herein by this reference. The processes also include homogenous catalysts as referred to in Hydrocarbon Processing (February 2006) pp 87-92, the contents of the entirety of which is incorporated by this reference. Other processes include U.S. Pat. Nos. 4,401,823; 5,354,914; 6,291,725 and 6,479,713, each of the contents of the entirety of which are incorporated by this reference. U.S. Pat. No. 6,479,713 describes a process including: substrates such as glycerol, sorbitol, xylitol, lactic acid, and arabinitol which are subjected to hydrogenolysis over a catalyst comprising Re—Ni supported on carbon at 230° C. and 1300 psi hydrogen pressure to give two- and three-carbon glycols similar to those obtained from petrochemical-based feed stocks. In one example, after 1 hour, 25.3% glycerol conversion was achieved with 72.3% selectivity to propylene glycol. The catalytic hydrogenolysis process may also involves a Nickel-on-alumina catalyst (C46-8-03 RS) obtained from United Catalysts Inc. (now Sud Chemie). This catalyst had the characteristics outlined in Table 4.
TABLE 4C46-8-03 RS Hydrogenolysis catalyst characteristicsNickel (wt %)48.9SiO2 (wt %)4.59Al2O3 (wt %)30.3ShapeCylindricalAverage length (mm)5.1Average crush strength (lbs/mm)1.8Reduction (%)43
The composition of the hydrogenolysis product mixture may be dependent on certain conditions, such as, for example, the particular bio-derived polyol feedstock or the hydrogenolysis process used. In a process (U.S. Pat. No. 6,479,713), mixed polyols were synthesized by feeding a 25% sorbitol solution into a reactor containing an alumina-based massive nickel catalyst (cylinder shaped) promoted with sodium hydroxide or sodium carbonate to 1% sodium. Over a period of 72 days, the feed (specific gravity, 1.1 g/mL, pH˜11.5) was fed into the reactor held at 180-250° C. under 200-1800 psi pressure. A representative product contained 47% propylene glycol, 20% ethylene glycol, 21% glycerol, and the remainder was mixed diols.
Hydrogenolysis is a fixed bed catalytic process that uses hydrogen from 1000-2000 psi, often at temperatures of 180-250° C. and typically is under alkaline conditions. U.S. Pat. No. 6,479,713 describes a process where a nickel-rhenium-on-carbon catalyst was loaded into a 300-mL semi-batch Parr reactor and purged with nitrogen. The catalyst was activated by adding hydrogen at 500 psi and heating to 280° C. for 16 h with stirring. The reactor was cooled, the hydrogen removed, and 105.5 g of an aqueous solution of sorbitol (25%) and KOH (0.94%) was added. The reactor was pressurized to 600 psi with hydrogen and heated; when the temperature reached 220° C., the pressure was raised to 1200 psi. The reaction was run for 4 h. Depending on the catalyst composition, sorbitol conversions ranged from 48.8 to 62.8%. In most cases, the major products were glycerol, propylene glycol, and ethylene glycol. Other feed stocks tested included xylitol, arabinitol, lactic acid, and glycerol.
In U.S. Pat. No. 6,479,713, alditols (such as a 15-40 wt % sorbitol solution in water) are catalytically hydrocracked in a fixed bed catalytic reaction process using an active nickel catalyst to produce at least about 30 W % conversion to glycerol and glycol products. The feed stream pH is controlled to between 7 and 14 by adding an alkali material such as calcium hydroxide or a strong base such as sodium hydroxide to prevent damage to the catalyst. Useful prior art reaction conditions are 400-500° F., 1200-2000 psig hydrogen partial pressure, and a liquid hourly space velocity of 1.5 to 3.0. To maintain desired catalyst activity and product yields, the catalyst is regenerated to provide catalyst age within the range of 20-200 hours. The reaction products are separated in distillation steps at successively lower pressures, and unconverted alditol feed is recycled to the reaction zone for further hydrogenolysis to produce 80-95 W % glycerol product. Sorbitol conversion is maintained at between about 30-70 W % by catalyst regeneration following 20 to 200 hours use, comprising washing to remove deposits and heating with hydrogen at 500-600° F. temperature. Countercurrent flow of feed and hydrogen in the reaction zone can be used if desired, particularly for achieving higher conversion of alditol feed to glycerol products.